Astronomers using data from the Euclid space telescope have discovered an exceptionally faint stellar system near the Fornax dwarf galaxy. Called Fornax-7, it appears to contain only about 170 times the mass of the sun in stars. The discovery paper was posted to the arXiv preprint server on Sept. 13.
Putting theories to the test
The lambda cold dark matter model of the universe predicts that dark matter halos of any mass should host their own smaller subhalos. This is well established for galaxies with masses similar to the Milky Way’s, which have many dwarf satellites, but it has never been confirmed for hosts much less massive than the Milky Way.
The current record-holder is the Large Magellanic Cloud (LMC), which has candidate ultra-faint companions. Finding a satellite around something as small as the Fornax dwarf spheroidal galaxy (Fornax dSph), roughly 100 times less massive than the LMC, would extend “the observed hierarchical formation of satellite systems nearly two orders of magnitude below the LMC,” researchers write in the paper.
Fornax is a dwarf galaxy that orbits the Milky Way, roughly 470,000 light-years away. It hosts six known globular clusters. Five of them are more than 10 billion years old but are spread far from the galaxy’s center. Given their mass and age, calculations predict that such clusters should have migrated toward the center by now.
This “timing problem” has fueled debate over whether Fornax’s dark matter halo is a diffuse, low-density “core” rather than the dense central “cusp” predicted by standard cold dark matter models.
The team, led by Teymoor Saifollahi of the Observatory of Strasbourg, serendipitously spotted a tiny, extremely faint stellar system called Fornax-7 in new Euclid satellite images. The system is very close to Fornax dSph.
A faint find
To pin down Fornax-7’s properties, the team first built a color-magnitude diagram from its six brightest, most reliable member stars, filtering out foreground Milky Way stars using Gaia motion data and color cuts. Six stars are not enough to establish the system’s age, metal content and distance, so the researchers simulated thousands of possible stellar populations and compared each with the data to find the best fit.
They then conducted more calculations to check whether Fornax-7 behaves like a star cluster or a dark matter-dominated galaxy.
The team’s models suggest Fornax-7 sits at roughly the same distance from Earth as the Fornax dSph, strongly hinting that the two are neighbors. In the sky, Fornax-7 is separated from Fornax dSph’s center by a projected distance of about 2.8 kiloparsecs (roughly 9,100 light-years), but researchers are not yet sure whether the two are gravitationally bound.
“This strongly suggests that Fornax-7 is located at the distance of the Fornax dSph,” the team says. “However, establishing whether the system is gravitationally associated with the galaxy will require follow-up radial-velocity measurements.”
‘Satellite of a satellite’
The system is extremely small and roughly 10 billion years old. It has very low mass, containing only about 170 solar masses of stars. “Our results show that Fornax-7 is among the faintest and smallest systems ever identified,” the team writes.
The team presents two scenarios: Fornax-7 could be a star cluster with no dark matter or a dark matter-dominated satellite galaxy orbiting Fornax dSph. If it is a star cluster, its properties could help resolve the “timing problem” and reveal the nature of its host galaxy’s dark matter distribution. If it is a dwarf satellite galaxy of Fornax dSph, it would be the “first known example of a ‘satellite of a satellite’ around a host galaxy as low in mass as Fornax,” researchers conclude.
Distinguishing between the scenarios requires measuring the system’s internal velocity dispersion through spectroscopy, likely using next-generation telescopes, researchers say. A third possibility is that it is an unrelated Milky Way halo object aligned by chance.
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Publication details
Saifollahi et al, Euclid: Data Release 1 (DR1) — Fornax-7, an ultra-faint companion to the Fornax dwarf spheroidal galaxy? A remote star cluster or dwarf satellite of the Fornax dSph, arXiv (2026). DOI: 10.48550/arxiv.2609.14535
Journal information:
arXiv
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A tiny stellar system may be a ‘satellite of a satellite’ (2026, September 28)
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